The invention relates to a power tool which comprises a housing, an output shaft connected a rotation motor to via an angle drive, wherein the angle drive comprises a drive spindle with a pinion, and a bevel gear connected to the output shaft, and a thrust bearing axially supporting the drive spindle relative to the housing.
In power tools of this type there is a problem related to a limited service life of the angle drive. Particularly, this problem occurs in power wrenches where relatively heavy torque loads are to be transferred. This is mainly due to the fact that the angle drive, gears and bearings together are of a small size in relation to a heavy transferred torque load, which means that the contact pressure on the gear teeth flanks is very high. In order to cope with this high contact pressure it is of greatest importance that the gear teeth engagement between the driving pinion and the bevel gear is absolutely correct under all load conditions, of course also that the gears are of a suitable material properly heat treated and machined.
To illustrate the magnitude of the challenge in seeking to solve the above mentioned problem a comparison can made with a gear box of a car where the service life of the gears in the gear box would be reduced to less than 1% of its normal service life if they were stressed to the same level as in a power wrench.
To obtain an accurate engagement between the pinion and bevel gear teeth the play between the gears is crucial. This play depends mainly on the axial position of the pinion and, hence, the drive spindle. The acceptable tolerance for the axial position of the pinion relative to the bevel gear is no more than ±0.01 mm. A common way of adjusting the pinion position is to use shims, but that technique is cumbersome and not accurate enough. The multiple contact surfaces between the shims, the drive spindle bearing and the housing also add to a slightly resilient support of the bearing and an accuracy of ±0.01 mm of the drive spindle and pinion position is not at all guaranteed. Moreover, the shims technique is rather time consuming and not suitable in mass production.
Another way to adjust a bearing play is to use threaded supported elements for the axial fixation of one of the ball races. That solution, however, contains another difficulty, namely that the ball race must not rotate relative its support during operation but still be easily movable during adjustment. These two contradictory features are very hard to accomplish.